Insights Into the Chromatin Structure of Thermoplasma volcanium: Archaeal HU Regulates Alba-Mediated DNA Compaction

Archaeal species encode a diversity of chromatin proteins that play distinct roles in genome compaction. Although our understanding of the individual proteins has been growing, their contributions to higher-order folding in the archaeal chromosome remain to be explored. In this study, we investigated the chromatin structure of Thermoplasma volcanium, a euryarchaeon that encodes a heat-unstable (HU) family protein (HUTvo) instead of histones. Atomic force microscopy revealed that the T. volcanium chromatin contains nucleoprotein structures of progressively increasing sizes, from 20nm filaments to ~50nm globules, indicative of higher-order DNA folding. In vitro reconstitution showed that HUTvo wraps DNA similar to histones and forms highly compact yet disorganized nucleoprotein clusters on its own. Remarkably, HUTvo only dampens the formation of higher-order structures by Alba. Our results provide an insight into how the interplay of chromatin proteins regulates genome compaction in histone-free archaea.